This invention relates to a process for applying a coating dispersion to a surface of a support member and more specifically to an extrusion or slot coating process for applying a ribbon-like stream of coating material to a substrate.
Numerous techniques have been devised to form a layer of a coating composition on a substrate. One of these techniques involves the use of an extrusion die from which the coating composition is extruded onto the substrate. For fabrication of web type, flexible electrophotographic imaging members, the extrusion die must lay down very thin coatings meeting extremely precise, critical tolerances in the single or double digit micrometer ranges. During the extrusion or slot coating of thin layers, the window of operating parameters is extremely small and are affected by factors such as coating thickness, speed of substrate, Theological properties of coating liquids, vacuum pressure, relative speed of the ribbon of coating material, pressure applied to the coating material as it progresses through an extrusion nozzle, and the like.
Extrusion techniques for forming thin layers are known and described, for example in U.S. Pat. No. 4,521,457 and U.S. Pat. No. 5,614,260, the entire disclosures thereof being incorporated herein by reference. The extrusion die usually comprises spaced, walls or lands, each having a flat surface parallel to and facing the other. These spaced lands form a narrow, elongated, extrusion passageway having an entrance slot at one end and an exit slot at the opposite end of the passageway. The passageway normally has side walls to direct the flow of a thin ribbon shaped stream of coating composition. Generally, the coating composition is supplied by a reservoir or manifold positioned along the length of the entrance slot of the extrusion passageway. The coating composition liquid travels from a pump through a feed channel, such as a pipe, to the manifold of the extrusion die. The coating composition liquid is distributed by the manifold into the entrance slot of the extrusion passageway. The coating composition liquid then travels through the extrusion passageway and out the exit slot onto a substrate to be coated. A typical photoreceptor extrusion die manifold has a cavity in the shape of a cylinder having a straight imaginary axis. This cylindrical cavity has a constant cross sectional area from one end of the cavity to the opposite end. The feed channel or feed pipe is connected to the manifold cavity midway between the opposite ends of the cavity. The feed channel has an imaginary axis which is perpendicular to the imaginary axis of the cylindrical manifold cavity to form a "T" shaped configuration. The coating composition liquid supplied by the feed channel is distributed by the manifold to an extrusion passageway connected to the manifold. The extrusion passageway conveys the coating material liquid from the manifold and shapes it into a thin ribbon-like extrudate which is thereafter deposited as a coating onto a substrate. After various layers are deposited, the coated photoreceptor web is subsequently sliced to form rectangular sheets which are formed into a belt type photoreceptor by welding opposite ends of the sheet together.
Generally, variations in pressure applied to a charge transport layer coating solution as it progresses through an extrusion coating system does not affect the quality of the final coating significantly. Similarly, many dispersions such as inorganic particles (e.g., trigonal selenium particles) dispersed in a solution of film forming binder material are not affected by variations in pressure applied to it as the dispersion progresses through an extrusion coating system. When a conventional extrusion die is utilized for forming very thin coatings of dispersions of organic photoconductive particles, it has been found that defects resembling brush marks often appear along each edge of the deposited coating. These brush marks remain as defects in the dried coating and ultimately print out as undesirable artifacts in the final electrophotographic copy.
The coating materials for the charge generation layer of the photoreceptors are made of dispersions. The dispersion is non-Newtonian, which shows behaviors of shear thinning, thixotropy, and yield stress. The dispersion shows little or no deformation up to the yield stress. This leads to flocculation of dispersion particles and defects in the coated film.